1. Field of the Disclosure
Embodiments herein relate generally to oligomerization of isoolefins. More specifically, embodiments disclosed herein relate to processing of mixed C4 hydrocarbon streams to produce oligomers of isoolefins and linear butene recovery.
2. Background
Olefins produced in various refining operations, such as a steam cracker or an FCC, can be used as a valuable feedstock for gasoline blending. Depending on the source, olefins can vary in size, degree of branching, and position of the double bond. In some processes, olefins may be available as a mixed stream containing straight and branched olefins of various length and double bond position together with paraffins, dienes and acetylenes.
In order to meet the fuel blending requirements, such as octane rating or vapor pressure requirements, smaller olefin molecules must be upgraded to produce longer chain molecules. One commonly used method of upgrading smaller olefin molecules, such as C2 to C5 olefins, is an oligomerization reaction.
Oligomerization reactions involve contacting an olefin with a catalyst in order to produce a longer chain molecule. An oligomer can consist of two or more constituent olefin molecules. For example, dimerization is a type of oligomerization reaction that is limited to a combination of only two olefin molecules. If the olefin feed contains only one type of olefin, a dimer product is formed. If the olefin feed contains two or more different olefins or olefin isomers, a codimer product may also be formed.
Specifically, C4 olefin dimerization is widely used for producing isooctene, an intermediate that can be hydrogenated to produce isooctane, a high-value gasoline blending additive. Several representative olefin dimerization reactions are shown below:

A gas phase olefin oligomerization process is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502, where C2 to C5 olefins, fed as either pure olefins or in admixture with paraffins, are oligomerized via contacted with a zeolite fixed catalyst bed. Other oligomerization processes are disclosed in, for example, U.S. Pat. Nos. 4,242,530, 4,375,576, 5,003,124, and 7,145,049, among others
In any type of oligomerization reaction, the oligomerization catalyst activity can be drastically reduced due to poisoning, fouling, and coking frequently caused by impurities present in the olefin feed stream. Furthermore, various additives and impurities that may be present in the olefin feed can participate in side reactions leading to formation of undesirable byproducts. For example, the presence of normal butene in the isobutene oligomerization process to produce isooctene dimer can lead to formation of undesirable C8 codimers. Formation of C8 codimers can adversely affect an operator in two major ways. First, it reduces the effective yield of the C8 dimer target product, thus increasing the dimerization reactor feedstock and operating costs. Second, it may require additional costs associated with separation and removal of C8 codimers from the C8 dimer product.
Oligomerization reaction additives, such as a reaction moderator, can also participate in undesirable side reactions with the olefin or with the dimerization product. Moderator is frequently added to the oligomerization reaction in order to increase the dimer selectivity by limiting the extent of oligomerization reaction to the dimer stage. Suitable moderators include oxygenates, such as water, primary, secondary and tertiary alcohols and ethers. However, as a trade-off to achieving high dimer selectivity, a portion of the moderator can react with an olefin or a dimerization product to form heavy oxygenates, for example, MSBE. A representative reaction of an olefin with a moderator to form a heavy oxygenate is shown below:

Similar to other types of side reactions, the reaction of moderator to produce heavy oxygenates, such as MSBE, can also reduce the C8 dimer product yield and require additional separation costs in order to maintain the desired product purity.
Accordingly, there still exists a need for an improved methods for producing oligomers of isoolefins.